(Table 5). Externally, Wallaconchis melanesiensis cannot be distinguished from grey or black specimens of other Wallaconchis species. Internally, the combination of a narrow penis, an apple-shaped spermatheca, and a free oviduct (not attached to the body wall by fibers) distinguishes W. melanesiensis from all Wallaconchis species except W. comendadori, from which it differs by a shorter penis and a less convoluted deferent duct (in the anterior copulatory apparatus).

Color and morphology of live animals

(Fig. 60). The dorsal notum is generally grey, but may be blackish red. The ocular tentacles are dark grey. The hyponotum is light grey. The foot is yellow-orange.

External morphology.

Between eight and ten papillae bear dorsal eyes (three or four per papilla). There is a retractable papilla with eyes in the center of the dorsal notum, which is not raised above the other papillae.

Digestive system

(Figs 61-62, Table 4). Examples of radular formulae are presented in Table 4. The length of the rachidian teeth is approximately 20 µm, significantly smaller than that of the lateral teeth. The length of the hook of the lateral teeth gradually increases (from 30 to 50 µm) from the inner to the outer teeth (excluding the innermost and outermost lateral teeth which are significantly smaller). The intestinal loops are of type I.

Reproductive system

(Fig. 63). The oviduct is narrow (approximately the same width as the deferent duct). The spermatheca is apple-shaped, with two lobes, and joins the oviduct through a short duct.

Copulatory apparatus

(Figs 64-65). The penis (from 0.5 to 1 mm long) is extremely narrow (approximately 20-30 µm) and smooth with no hooks (Fig. 65). The penial sheath is narrow proximally and widens distally into a vestibule (of which the shape varies) (Fig. 64). The penis is within the proximal region of the vestibule, i.e., near the end of the penial sheath (Fig. 64A). The deferent duct is highly convoluted. The length of the penial sheath is approximately two thirds of the body cavity. The deferent duct is thicker than the penial sheath (excluding the vestibule). The retractor muscle is narrow and inserts at the posterior end of the body cavity, near the rectum.

The application of Onchidium cinereum Quoy & Gaimard, 1832 (with a type locality in Tonga) has remained confusing. The original description is short and uninformative. The type material was not located. At this stage, it cannot be determined whether Onchidium cinereum applies to a species of Peronia, Wallaconchis, or another genus. Therefore, Onchidium cinereum is regarded here as a nomen dubium. Semper re-described O. cinereum based on Tonga specimens from the collections of the Museum Godeffroy (not part of the type series), and these specimens are part of a Wallaconchis species (based on the anatomical characters mentioned). The specimens that Semper examined indicate that a Wallaconchis species lives in Tonga, which could be W. comendadori, W. melanesiensis, or even a distinct species, but this could not be tested here because we did not have access to Tonga material. Finally, note that Hoffmann (1928) and Labbé (1934) commented on O. cinereum based on Semper’s re-description, not based on the original descriptiotion or new material.

Intra-specific genetic divergence is higher in W. melanesiensis than in other Wallaconchis species. Specimens from Vanuatu are 3.8% to 5.6% genetically divergent from the other specimens (from Indonesia and Papua New Guinea). High genetic divergences are even observed between specimens from the same locality (e.g., 3.4% within Vanuatu and Halmahera, and 4.4% within Kavieng, Papua New Guinea). The genetic divergence between the individuals from Vanuatu and those from Papua New Guinea and Indonesia could simply be an artifact of the geographic isolation of Vanuatu. Also, intra-specific divergences up to 5.5% were observed within other onchidiid species ( Dayrat et al. 2016). The presence of a distinct species in Vanuatu cannot be excluded but would result in splitting W. melanesiensis into three or more species, as the specimens from Papua New Guinea and Indonesia are not reciprocally monophyletic with respect to Vanuatu individuals; therefore, specimens from Vanuatu are not currently considered to be a distinct species.